"a long time ago", a law was passed banning the importation of milsurp firearms. That law is called the Gun Control Act of 1968. It was written by lawyers for Winchester. (It has since been modified, but the intent was clear at the time.)
The fact is that even a quality milsurp rifle, like a '98 Mauser, modified to be the equivalent of a good sporting rifle, will in the end cost as much or more than the sporting rifle, unless the owner is qualilfied to do most of the work himself.
Will a converted military rifle be superior to a modern sporting rifle? No. Will it be as good? Maybe, if the rifle started as a quality Mauser or Springfield 1903. But the person who spends $800-2000 dollars making a sporting rifle out of a Mosin-Nagant will still have a Mosin-Nagant, worth maybe $150.